Multi-channel arrangement projection systems are projection systems projecting a composite image which is subdivided in two or more sub-images each projected by means of a separate projector element, the sub-images being tiled in such a way that together they form a bigger image. The advantage of this method of projection is that the field of view can be increased while maintaining a high image resolution and that a system with small footprint can be used for projecting large images. This kind of large field of view projection systems serve different broad areas of application: electronic presentations, e.g. for business, education and advertising; entertainment applications, e.g. home theatre, electronic cinema; status and information presentations, e.g. military, utilities, transportation; simulation applications in e.g. training and games.
When tiling sub-images of a tiled image, e.g. projected from a multi-channel projection system, hard edge or soft edge techniques can be applied. Where hard edge techniques are applied, the projected sub-images are put next to each other. This is often not an optimal solution because the edges between the sub-images are difficult to arrange in perfect alignment and the join may well be noticeable. Where soft edge techniques are applied, an overlap area is created in which two or more neighboring sub-images, i.e. at least a first sub-image and second sub-image, are blended so that there is at least a gradual transition from the first sub-image to the second sub-image. Therefore the first sub-image is progressively spatially faded out whilst the second sub-image is progressively spatially faded in. The principle of soft edging is illustrated by means of FIG. 1. In FIG. 1, a first sub-image 2 and a second sub-image 4 are projected by means of a first projector 6 and a second projector 8, respectively, in such a way that an overlap area 10 is created in the image 12 formed by the first sub-image 2 and the second sub-image 4 on the screen, namely in the area which is illuminated by more than one projector, in the present case by the first projector 6 and the second projector 8. In case of a good soft edge, the first and second sub-images 2, 4 are processed, optically or electrically, in such a way that the sum of the light intensity in the overlap area 10 is equal to the average intensity outside the overlap area 10, and in such a way that the contribution of the first projector 6 to this total intensity in the image 12 changes gradually from maximum to zero from a first side of the overlap area 10 to a second side of the overlap area 10, while the contribution of the second projector 8 to this total intensity changes gradually from zero to maximum from the first side of the overlap area 10 to the second side of the overlap area 10.
Two types of soft edge techniques or blending techniques are known: electrical blending and optical blending. The use of electrical edge blending is known in the industry and is widely used in cathode ray tubes (CRT), digital light processing displays (DLP™), liquid crystal displays (LCD) and other projection display technologies. The advantages of electrical blending are known: real time control, dynamically changeable based upon imagery or changes in the system configuration, flexible attenuation curves of any type can easily and simply be defined. Nevertheless, electronic blending typically suffers from “double black” errors, i.e. whereby the combination of light leakages in an overlapping blend region leads to a black level having an intensity that is twice the level of the black level in non-overlapping areas.
Optical blending, using optical masks either with hard edges, dither patterns, or gradient patterns of some kind whereby the devices are placed either internally or externally to the projectors to cause smooth transitions in blend regions to produce a satisfactory blend also are known. Such systems are described for example in WO 95/25292 and WO 01/41455. Optical blending systems do not suffer from the “double black” problem and offer an acceptable blend at all brightness levels from white to black, which often makes it the most preferable blending technique. Nevertheless, changes to the blend regions e.g. for the purposes of changes in display configuration are not possible without articulated mechanical systems.
Adjustment of the blending of sub-images in a multi-channel projection system typically is performed only during set-up and calibration of a multi-channel projection system or during maintenance. Typically each optical blending means needs to be adjusted with respect to its neighbors, whereby an optimized result is obtained by an iterative adjustment. The latter thus requires that at each projector the blending means is positioned optimally, which is time-consuming. Furthermore, in many of today's systems, adjustment of optical blending is performed manually, making it very tedious and labor-intensive. As evaluation of the quality of the blend preferably is performed viewing the viewing side of the screen—the rear-side typically has a different emission characteristic—blending adjustment typically requires more than one person.